Apparatus and method for synthesizing films and coatings by focused particle beam deposition

ABSTRACT

A particle beam deposition apparatus includes a particle source for generating a plurality of particles in suspended form, an expansion chamber, and a deposition chamber connected to the expansion chamber by an aerodynamic focusing stage, and containing a substrate. The aerodynamic focusing stage may be comprised of a plurality of aerodynamic focusing elements, or lenses. Particles, including nanoparticles, may be deposited on the substrate by generating an aerosol cloud of particles, accelerating the particles into the expansion chamber, creating a collimated beam out of the particles by passing them through the aerodynamic focusing lenses and into a deposition chamber, and impacting the particles into the substrate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a National Stage application under 35 U.S.C. §371and claims benefit under 35 U.S.C. §119(a) of International ApplicationNo. PCT/US01/22766 having an International Filing Date of Jul. 19, 2001,which claims benefit of U.S. Provisional application Ser. No. 60/219,728filed on Jul. 19, 2000.

STATEMENT AS TO FEDERALLY-SPONSORED RESEARCH

Funding for work described herein was provided in part by the federalgovernment, which may have certain rights in the invention.

TECHNICAL FIELD

This invention relates to deposition of particles on a substrate, andmore particularly to deposition of nanoparticles by focused particlebeam deposition to form films or coatings.

BACKGROUND

The synthesis and processing of nanostructured materials, i.e.,materials with grain sizes less than about 100 nm, is of great interestbecause such materials are known to have properties different from, andoften superior to, those of conventional bulk materials. Theseadvantages include greater strength, hardness, ductility, andsinterability, size-dependent light absorption, and greater reactivity.Applications for these advanced materials include ductile ceramics,wear-resistant coatings, thermal barrier coatings, new electronic andoptical devices, and catalysts. There has been considerable progress indetermining the properties of nanostructured materials, small amounts ofwhich have been synthesized (mainly as nanosize powders), by processessuch as colloidal precipitation, mechanical grinding, and gas-phasenucleation and growth. The focus of more recent research has been toproduce nanostructured materials directly in a form suitable for use ina practical application, such as wear-resistant coatings. In suchmaterials, the nanostructure is deliberately introduced to takeadvantage of superior properties, for example, enhanced hardness in thecase of nanostructured wear-resistant coatings.

The current interest in nanostructured materials has led to the searchfor methods to synthesize such materials in the form of bulk solids orfilms. The production of nanostructured materials has generally involvedtwo or more steps, including the controlled synthesis of nanosizepowders, and the assembly of these powders into nanostructured materialsby sintering or other means.

Gas-phase nucleation and growth of particles is an established route forthe synthesis of nanosized powders and includes such techniques asevaporation-condensation, laser pyrolysis, and thermal plasma expansionprocessing. Gleiter, H., “Nanocrystalline Materials,” Prog. Mater. Sci.33:223-315 (1989); Recknagle, K. et al., “Design and Operation of aNanocluster Generation and Collection System,” Aerosol Sci. Technol.22:3-10 (1995); Oda, M. et al., “Ultrafine particle films by gasdeposition method,” Mat. Res. Soc. Symp. Proc. 286:121-130 (1993);Flint, J. H. et al., “Powder temperature, size and number density inlaser-driven reactions,” Aerosol Sci. Technol. 5:249-260 (1986); Rao, N.et al., “Nanoparticle formation using a plasma expansion process,”Plasma Chem. Plasma Proc., 15:581-606 (1995). In many of these gas-phaseprocesses, the nanosize powders are collected thermophoretically andconsolidated in-situ using high pressure compaction to produce pelletsof nanostructured materials. Gleiter, H., “Nanocrystalline Materials,”Prog. Mater. Sci. 33:223-315 (1989); Recknagle, K. et al., “Design andOperation of a Nanocluster Generation and Collection System,” AerosolSci. Technol. 22:3-10 (1995).

The use of inertial impaction provides a convenient route for assemblingparticles into consolidated materials, including nanoparticles intonanostructured materials. That is because heavy particles seeded in alight carrier gas can be deposited efficiently by accelerating the gasthrough a nozzle, preferably into a low pressure region, and directingthe resulting high speed aerosol jet against a deposition substrate.Fernandez de la Mora, J., “Surface impact of seeded jets at relativelylarge background densities,” J. Chem. Phys. 82:3453-3464 (1985);Fernandez de la Mora et al., “Hypersonic impaction of ultrafineparticles,” J. Aerosol Sci. 21:169-187 (1990). At sufficiently lowpressures, even though the host gas may decelerate before impacting thesubstrate, e.g., by formation of a shock, the heavy particles continuetheir forward motion and impact by virtue of their greater inertia.

Until recently, high-speed impaction had been used mainly for particlemeasurement. More recently, however, a number of materials depositionprocesses based on this principle have been developed, including thosethat deposit heavy molecules, ultra fine particles, and largemicron-sized particles. Schmitt, J. J., “Method and apparatus for thedeposition of solid films of a material from a jet stream entraining thegaseous phase of said material,” U.S. Pat. No. 4,788,082 (1988);Halpern, B. L. et al., “Gas jet deposition of thin films,” Appl. Surf.Sci. 48/49:19-26 (1991); Calcote, H. F. et al., “A new gas-phasecombustion synthesis process for pure metals, alloys, and ceramics,”24th Symp. (Intl.) on Combustion, Combustion Inst. Pittsburgh 1869-76(1992); Kashu, S. et al., “Deposition of ultrafine particles using a gasjet,” Jap. J. Appl. Phys. 23:L910-912 (1984); Oda, M. et al., “Ultrafineparticle films by gas deposition method,” Mat. Res. Soc. Symp. Proc.286:121-130 (1993); Gould, R. K. et al., “Apparatus for producing highpurity silicon from flames of sodium and silicon tetrachloride,” U.S.Pat. No. 5,021,221 (1991).

Some of the inventors of this disclosure have themselves participated indeveloping a process, hypersonic plasma particle deposition (HPPD), forthe inertial deposition of nanoparticles to form nanostructured films.In HPPD, vapor-phase reactants are injected into a thermal plasma, whichis then expanded to low pressure through a nozzle. Rapid cooling in thenozzle expansion drives the nucleation of nanoparticles, which are thenaccelerated in the hypersonic free jet issuing from the nozzle. Asubstrate may be positioned normal to the flow, and particles as smallas a few nanometers in diameter deposit by inertial impaction. Ballisticcompaction forms a dense, nanostructured coating. In experimentsinvolving silicon carbide deposition, the grain size observed byscanning electron microscopy (SEM), typically around 20 nm, correspondedclosely to measurements by scanning electrical mobility spectrometry ofthe aerosol sampled in-flight downstream of the nozzle, indicating thatthe film retained the grain size of the impacting particles. Rao, N. P.et al., “Nanostructured materials production by hypersonic plasmaparticle deposition,” Nanostructured Materials, 9:129-132 (1997); Rao,N. P. et al., “Hypersonic Plasma Particle Deposition of NanostructuredSilicon and Silicon Carbide,” J. Aerosol Sci., 29:707-720 (1998); Rao,N. et al., “Plasma chem. Plasma Process 15, 581 (1995); Neumann, A. etal., “J. Nanoparticle Res., accepted for publication January 1999; Blum,J. et al., “Nanoparticle Research,” 1, 31 (1999). A patent on the HPPDprocess, U.S. Pat. No. 5,874,134, assigned to the Regents of theUniversity of Minnesota, is hereby incorporated by reference.

Nanoparticle deposition processes have recently been used to createpatterned films without the use of masking through the use of collimatedbeams of nanoparticles. Most of this work has been directed atdepositing metal patterns for printed circuit board and electronicsapplications. Schroth, A. et al., Jpn. J. Appl. Phys., 37, 5342 (1998);Akedo, J. et al., Jpn. J. Appl. Phys., 38:5397 (1999); Akedo, M. et al.,“Sensors Actuators,” A 69:106 (1998). For example, the gas jetdeposition (GJD) process has been used to “write” metal patterns fordepositing gold or silver particles generated by inert gas-condensation.Hayashi, C. et al., “The use of nanoparticles as coatings,” MaterialsSci. Eng. A163:157-161 (1993). In this method, condensable vapor isgenerated above a heated crucible and particles nucleate in an inertcarrier gas. The particle-laden flow then expands supersonically througha micronozzle, producing a particle beam whose dimensions areapproximately the same as, or perhaps somewhat smaller than, those ofthe nozzle. Nozzles with inside diameters of 100 μm were used to depositgold particles, producing tapered needle-shaped structures. Severaldeposition nozzle designs were tested, including rectangular slit-typegeometric and multinozzle assemblies.

Similar techniques have been used to create patterned films bydepositing iron nanoparticles, and more recently to fabricate highaspect ratio structures for micro-electro-mechanical systems (MEMS)applications. A variety of techniques were used, including free forming,insert-molding, and substrate masking. To suppress clogging, the nozzleswere heated, so as to drive particles away from the nozzle walls bythermophoretic forces. Representative techniques are described inKizaki, Y. et al., “Ultrafine Particle Beam Deposition I. Sampling andTransportation Methods for Ultrafine Particles,” Jpn. J. Appl. Phys.Vol. 32:5163-5169 (1993); Akedo, J. et al., “Fabrication of ThreeDimensional Micro Structure Composed of Different Materials UsingExcimer Laser Ablation and Jet Molding,” Proceedings of the 10th AnnualInternational Workshop on Micro Electro Mechanical systems, Nagoya,Japan, 135-140, Jan. 26-30, 1997.

Although the GJD process has aspects similar to HPPD, the processconditions used in GJD are significantly different, with a relativelyhigh particle source chamber pressures (−0 Torr −5 atm.), necessitatingthe use of light carrier gases (e.g., helium) and small nozzledimensions (−100 μm) to achieve inertial deposition of nanoparticles.Deposition rates for GJD are on the order of 10 μm/min, with depositdimensions close to that of the nozzle, though the sharpness of thepattern is somewhat limited by the presence of a broad “tail”surrounding the core deposit.

The GJD method is limited in its ability to produce very small features.Pattern feature dimensions have been decreased down to 40 μm by suitablyreducing nozzle dimensions. Reducing the nozzle size, however, greatlydiminishes the deposition rate, since the gas flow rate varies as thesquare of the nozzle diameter. In addition, the use of small nozzlesmakes it more difficult to control the nozzle-to-substrate distanceprecisely, which in the GJD process scales with the nozzle diameter anddetermines the cut-size of impacting particles. Smaller nozzles are alsodifficult to manufacture with precision and are more susceptible toclogging at high particle loading.

Particle beams in areas other than nanoparticles have been controlledand focused using aerodynamic lenses. Aerodynamic focusing is discussedby Dahneke, B. et al., “Similarity theory for aerosol beams,” J. ColloidInterface Sci., 87, 167-179 (1982); Fernandez de la Mora et al.,“Aerodynamic focusing of particles and molecules in seeded supersonicjets,” in Rarefied Gas dynamics: Physical Phenomena, [edited by Muntz,E. P., weaver, D. P. and Campbell, D. H.], Vol. 117 of Progress inAstronautics and Aeronautics, AIAA, Washington D.C., 247-277 (1989), andaerodynamic lenses are described by Liu, P. et al., “Generating particlebeams of controlled dimensions and divergence: I. Theory of particlemotion in aerodynamic lenses and nozzle expansions,” Aerosol Sci.Technol. 22:293-313 (1995); Liu, P. et al., “Generating particle beamsof controlled dimensions and divergence: II. Experimental evaluation ofparticle motion in aerodynamic lenses and nozzle expansions,” AerosolSci. Technol. 22:314-324 (1995). In aerodynamic focusing, particles maybe shaped into a narrow beam by passing the aerosol through a series ofconstrictions (aerodynamic lenses). The gas undergoes a converging anddiverging motion as it flows through the lenses. Due to inertia,particles seeded in the flow may either concentrate along the flow axisor be deposited on the walls of the focusing system, depending on theirsize. For a given lens geometry, gas composition, particle composition,and gas flow rate, this tendency to focus depends strongly on theparticle size. Particles of a certain critical size are pushed to theaxis, while subcritical particles show a concentrating tendency thatdecreases with particle size. Particles larger than critical overshootthe focal point and are removed from the flow by collision with walls.With a single lens, only particles within a very narrow range ofparticle sizes are focused. Particles within a broader size range can tobe focused by using a set of focusing lenses with gradually decreasingdiameters. Typically, five lenses may be needed to cover one order ofmagnitude in particle size. The lenses may be used in combination with adownstream supersonic nozzle to accelerate and deposit particles in aspecified range of particle sizes. Aerodynamic focusing is discussed inU.S. Pat. No. 5,270,542, incorporated herein by reference.

SUMMARY

A new technology is described herein for depositing patterned films andcoatings in general, and functional nanostructures such asnanostructured patterned films and coatings in particular. Thetechnology is based on the generation of gas-borne particles, such asnanoparticles from a thermal plasma expansion reactor, confining theparticles in a narrow, high-speed particle beam by passing the aerosolflow through an aerodynamic focusing stage, followed by high-speedimpaction of the tightly focused particles onto a substrate in a vacuumdeposition chamber. The focusing stage may consist of one or morefocusing. elements (aerodynamic lenses) and a nozzle that may beintegrated into the inlet of the deposition chamber. The aerodynamiclenses may focus the particles emerging from the aerosol source into anarrow beam, while the nozzle accelerates the focused particles toterminal velocities on the order of the sonic speed of the carrier gas.The particles in the beam may be deposited by inertial impaction ontothe substrate in the evacuated deposition chamber, resulting in theformation of a consolidated deposit. Patterned films may be deposited bytranslating the substrate with respect to the nozzle, or vice versa.

Nanostructured patterned films of various materials may be deposited inthis manner using an aerosol source wherein the generated particles arenanosized (i.e., with diameters<100 nm). Nanostructured patterns ofmaterials such as silicon carbide may be used in applications such aswear-resistant coatings for MEMS or micro-fluidics systems and devices.Line widths of less than 50 microns can be produced. This approach canpermit the use of much larger nozzles than in previously developedmicronozzle methods, and also can permit size-selection of the particlesthat are deposited.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram of a system for depositing nanoparticleson a substrate via a collimated beam.

FIG. 2 is a schematic diagram of another system for depositingnanoparticles on a substrate via a collimated beam.

FIG. 3 a is a diagram of predicted gas streamlines in a nanoparticlefocused beam deposition system.

FIG. 3 b is a diagram of predicted particle trajectories in ananoparticle focused beam deposition system.

FIG. 4 is a scanning electron microscope image of a needle-shapedsilicon carbide tower produced by nanoparticle focused beam deposition.

FIG. 5 is a scanning electron microscope image of a pattern of siliconcarbide produced by nanoparticle focused beam deposition.

FIG. 6 is a schematic diagram of a system for depositing nanoparticleson a substrate via a collimated beam.

FIG. 7 a is a diagram of predicted velocity vectors and Mach numbers ina hypersonic particle beam deposition system.

FIG. 7 b is a diagram of predicted temperature contours in a hypersonicparticle beam deposition system.

FIG. 8 is a graph comparing the RBS spectrum for a deposited film of SiCto a spectrum for standard SiC.

FIG. 9 is a graph that compares hardness for a deposited film of SiC tothat of standard SiC.

DETAILED DESCRIPTION

A novel technology, termed focused particle beam deposition (FPBD), iscapable of depositing patterned films, including nanostructuredpatterned films, with higher deposition rates and sharper definitionthan that possible with the prior art processes. One type of apparatusfor practicing FPBD is schematically depicted in FIG. 1. In theapparatus 10, gas-borne particles are generated in a source 12 such as aplasma expansion reactor. The particles are transported by the flow intoan intermediate expansion chamber 16, which is maintained at a pressure,typically below 10 torr, by a mechanical vacuum pump 18. The expansionchamber 16 may communicate with a downstream deposition chamber 20through a focusing inlet 24. A portion of the expanded flow from theaerosol source may enter the deposition chamber 20 through an inlet 24provided with aerodynamic focusing lenses 26 and an accelerating nozzle28. The deposition chamber 20 may be evacuated by a vacuum pump 30, suchas a turbo pump, and maintained at a low pressure, e.g., on the order of0.01 torr or lower in the case of a turbo pump.

The particles in the flow entering this final chamber 20 may be focusedinto a narrow beam and then impacted at high speeds against a substrate32 placed within the chamber 20, at a distance within 1-100 nozzlediameters downstream of the inlet 24. Patterned films may be depositedby translating the substrate 32 in relation to the nozzle, orvice-versa. The differential pumping arrangement of the type shown inFIG. 1 is flexible and may permit independent control of aerosol sourceflow and deposition flow rates. Flexibility in setting the pressure ofthe intermediate chamber may provide a means by which to apply the FPBDprocess over a broad range of particle sizes. In addition, the aerosolexpansion chamber could be omitted under certain conditions.

The focusing assembly 22 may consist of a series of thin plates 34mounted in a cylindrical barrel 36. Each plate 34 has an orifice, or“aerodynamic lens,” located at its center, and the assembly 22 mayterminate in an exit nozzle 24, which is a sonic orifice. In passingthrough each lens 26, the streamlines of the gas flow contract andre-expand. Very small particles follow the gas streamlines. Very largeparticles accelerate radially toward the axis as the flow approaches theorifice. Due to their high inertia, these particles are projected acrossthe centerline and impact on the opposite wall of the focusing assembly.Particles of an intermediate size are also accelerated toward the axis,but due to their shorter aerodynamic stopping distance, they terminatetheir radial motion on a flow streamline that is closer to the axis thanthe one on which they originated. This may lead to a concentration ofsuch particles along the axis. By careful design of lens assembly 22, itis possible to collimate particles having a specified range of sizes.

The use of aerodynamic lenses upstream of the nozzle may enable thedeposition of entrained particles within a tightly confined area whosediameter is substantially narrower than that of the nozzle, e.g., by anorder of magnitude or more. As a result, for equivalent nozzle sizes,the use of aerodynamic focusing may enable the deposition of patternfeatures with dimensions up to 10 sizes smaller than that possible withconventional gas jet deposition without the use of lenses.Alternatively, for equivalent feature dimensions and source particleconcentrations, the use of aerodynamic focusing can permit depositionrates (i.e., “pattern writing speeds”) 100 times greater than thatpossible in conventional gas jet deposition without focusing. Theminimum attainable beam diameter depends on practical design issues suchas tolerances in lens alignment and the residual thermal motion ofparticles exiting the nozzle. Small particles still retain a componentof thermal velocity in the radial direction which causes the focusedbeam to broaden downstream of the nozzle. The beam diameter increaseswith decreasing particle size and increasing beam path length. For abeam of 20 nm particles, the beam divergence over a nozzle-to-substrateseparation distance of 5 mm may be on the order of 10 μm. This isrelatively low, and it is likely that factors such as tolerances in lensalignment will have a greater effect on the minimum attainable featuresize.

In one embodiment of the system, shown in FIG. 2, particles in thesub-100 nm range may be generated in a nanoparticle source 40 comprisinga plasma expansion reactor. The in-flight agglomeration of the particlesmay be minimized by hypersonic transport into an intermediate expansionchamber 42. The particles may then be confined into a narrow beam bypassage through a focusing inlet 44 provided with aerodynamic lenses 46,communicated with a downstream vacuum deposition chamber 48, anddeposited onto a translating substrate 50. The nanoparticle source 40preferably comprises a dc torch 52 with a ceramic flow passage 54mounted downstream, into which reactant feedstock, preferably in gas- orvapor-phase, is injected. The pressure in the injection region ispreferably in the range 300-750 torr, but may be any appropriate value.Nanoparticles are nucleated by expanding the hot reacting gases througha ceramic-lined nozzle 56, preferably having exit diameters in the range1-10 mm. The gas temperature at the nozzle entrance is preferablybetween 2000K and 5000K, and that at the nozzle exit between 1000K and2500K. The expansion chamber is preferably at a pressure below 10 torr.and is pumped by a large Roots Blower (or equivalent) mechanical vacuumpump 58. Flow rates through the reactor are preferably in the range1-100 slpm.

The focusing inlet 44 communicating with the final deposition chamber 48may have multiple lens elements 46 to focus a broad range of particlesizes. In certain embodiments, it may be preferable to have three tofive lens elements. The inlet 44 may be provided with an acceleratingnozzle which may be operated under choked flow to accelerate theparticles previously focused. The diameters of the aerodynamic lenselements 46 are selected preferably to focus particles within thesub-100 nm diameter range. The diameters of the lens elements 46 and thenozzle preferably range from 0.1 to 5 mm. The actual values depend onprocess parameters such as carrier gas composition, expansion chambertemperature and pressure, and particle aerodynamic diameter. Thenozzle-to-deposition substrate distance is preferably in the range of1-100 nozzle diameters.

The system and process are not limited to sources comprising a thermalplasma reactor alone. The FPBD process is also not limited to particlesgenerated by chemical reaction alone, but is also applicable toparticles generated by physical evaporation and condensation, e.g.,starting from feedstock materials including solids, liquids, and gases;and by electrospray processing. The FPBD concept can be applied ingeneral to any source of gas-borne nanoparticles, including laserpyrolysis reactors, evaporation-condensation reactors, or electrosprayatomizers, in processes in which the object of processing is thedeposition of gas-borne nanoparticles onto substrates to form patternedfilms of various materials, such as metals, inorganic materials (e.g.,ceramic oxides, carbides, nitrides, and mixtures of the same), andorganic materials (e.g., polymers and plastics). The FPBD systemincorporating a nanoparticle source can be used to depositnanostructured patterned films, which may possess unique and enhancedproperties when compared to films of more conventional materials.

Experiments were conducted in which either SiC or titanium nanoparticleswere generated. An argon-hydrogen plasma was generated by adirect-current arc. Reactants were injected into the plasma at theupstream end of the expansion nozzle. The reactants consisted either ofsilicon tetrachloride and methane, for silicon carbide synthesis, ortitanium tetrachloride, for titanium synthesis. The pressure wasapproximately 50 kPa at the nozzle inlet and 345 Pa in the expansionchamber. The plasma is hot (approximately 2000 K) at the nozzle exit andexpands supersonically into the large expansion chamber. The inlet tubeto the aerodynamic lens assembly was coaxially located 75 cm downstreamof the plasma expansion nozzle. The flow in the expansion chamberexperienced a series of expansion and compression waves, and is expectedto have relaxed to close to room temperature at the inlet of the lensassembly.

The lens assembly consisted of a series of five lenses, each with anorifice diameter of 2.26 mm. The inner diameter of the exit nozzle was1.85 mm. Each lens was 0.3 mm thick, and the lenses were spaced 47 mmapart. The entire unit was constructed of stainless steel.

The particle beam exiting the lens assembly issued into a chamber thatwas maintained at a pressure of 1.0 Pa. Substrates were mounted in thischamber, typically 3 mm downstream of the exit nozzle. The particleimpact velocity was estimated to be in the range 200-300 m/s. Thesubstrates were at room temperature. Various substrate materials wereused, including stainless steel, aluminum, brass and glass. Adherentdeposits formed on all of the metal substrates, but not on glass.

Numerical simulations were conducted to predict the flow of carrier gasand the trajectories of (assumed spherical) silicon carbide particles ofvarious sizes through a lens system with the same geometry andconditions as in the experiments. These simulations solved theconservation equations for steady, laminar, compressible flow, andcalculated particle trajectories accounting for viscous drag but notBrownian diffusion. Brownian diffusion would be expected to broaden thewidth of the focused beam, especially for particles smaller than about10 nm in diameter. The predicted gas streamlines and trajectories of20-nm-diameter particles that enter the lens assembly along variousstreamlines are shown in FIGS. 3 a and 3 b, respectively. The particlesare predicted to be well collimated along the flow axis by the exit ofthe final lens.

An SEM image of a needle-shaped silicon carbide tower deposited on astationary substrate produced by FPBD is shown in FIG. 4. The height ofthis structure is 1.3 mm. The compact, tapered appearance is typical forboth SiC and titanium. High-resolution SEM images obtained fromcross-sections of the titanium deposits show grain sizes of about 20 nm,similar to those previously reported for HPPD of silicon carbide. Ingeneral, the height, half-width, and base diameters of the towersincrease linearly with deposition time, and the dimensions are similarto those using micronozzles. Kashu, S. et al., Jpn. J. Appl. Phys.23:L910 (1984); Oda, M. et al., MRS Symp. Proc. 286:121 (1993); Schroth,A. et al., Jpn. J. Appl. Phys. 37:5342 (1998); Akedo, J. et al., Jpn. J.Appl. Phys. 38:5397 (1999); Akedo, J. et al., Sensors Actuators A 69:106(1998). However, aerodynamic focusing may allow one to use much largernozzles to achieve the equivalent results. Because a ten-fold increasein nozzle diameter corresponds to a hundred-fold increase in flowrate,aerodynamic focusing may allow much higher throughputs. In addition,although nozzle clogging may still be an issue with the present system,the use of larger nozzles should help alleviate the problem.

Experiments have demonstrated the feasibility of depositing lines andtwo-dimensional patterns by translating the substrate. A pattern formedby SiC particles is shown in FIG. 5. The substrate was translatedmanually in a rapid zig-zag motion. The minimum line width is about 50μm. As can be seen in the figure, towers began to grow at several pointswhen the translation momentarily paused. An automated x-y translationsystem has also been implemented and has been used to deposit titaniumlines, with a width of about 30 to 50 μm. Lower Si precursor feeds inthe plasma reactor improve operation characteristics and produce cleanernanoparticle streams. FIGS. 4 and 5 are not very clear—image quality isvery poor.

Downstream of the lenses, a critical nozzle forces hypersonic depositionof the particles. The dimensions of that nozzle define the final featuresize of the patterns. Beams with diameters in the order of tens ofmicrons may be produced.

Another embodiment, which can be used at relatively low flow rates, isshown schematically in FIG. 6. Its principle component is ahigh-temperature plasma reactor 60 with an injection ring 62 and anozzle 64. The injection ring 62 and nozzle 64 may be constructed from asingle piece of boron nitride. Reactants (e.g., SiCl₄ and CH₄) may beinjected into the plasma, at a location where the temperature is highenough (e.g., >4000K) to promote virtually complete disassociation.Inside the nozzle 64, rapid quenching causes particles to nucleate.These particles may then be accelerated with the flow to supersonicvelocities. The reactant feed system 66 may include a bubbler system 68to gasify the SiCl₄ liquid precursor.

Substrate temperature control may be achieved by a cooling system (notshown) which may combine water and Ar/He feed, according to the designof Bieberich and Girshick. Bierberich, M. T., and Girshick, S. L.,Plasma Chem. Plasma Process. 16:157S (1996). A second chamber 70containing the substrate 72 may be pumped down to −10³ Torr by aturbomolecular pump 74, to assist in hypersonic expansion through thecritical orifice 76 located after the aerodynamic lenses 78.

Computations for a two-dimensional asymmetric flow of pure argon wereconducted. The computations simulate flow through the nozzle, includingdetailed heat transfer through the boron nitride nozzle walls, and theflow between the nozzle exit and the substrate. A uniform velocity inletcondition was assumed, and the gas temperature at the nozzle inlet wasset to 4000K. Other boundary conditions included a pressure of 2.5 Torrat the exit of the flow field, and convective and radiative heat lossesfrom the nozzle's outer boundary. The simulations were performed usingthe commercially available software CFD-ACE. A conjugate gradient solverwas used to make calculations across flow boundaries. A two-dimensionalplot of calculated velocity vectors and Mach numbers is shown in FIG. 7a, and predicted temperature contours are shown in FIG. 7 b. Machnumbers up to 6.5 are predicted. A detached recompression shock ispredicted to be located about 2.5 mm above the substrate.

While little materials characterization has been performed on the filmsdeposited by focused deposition, the properties are expected to be verysimilar to those obtained in the hypersonic plasma particle depositionprocess. These properties are described in the following.

In an SEM image, the deposited films appear to have the same morphologyas those deposited earlier using higher Si precursor flow rates, withthe exception of the absence of macroparticles in the new results.Rutherford back scattering (RBS) analysis showed that high substratetemperature (740° C.) reduced the chlorine content of the film to below1.5%. Film density was determined by RBS to be 80% of the theoreticalSiC density for the 740° C. substrate temperature experiments. Densitymeasurements indicated porosity increasing with decreasing substratetemperature. FIG. 8 shows a comparison of the RBS spectrum for adeposited film to a spectrum for standard SiC.

Mechanical properties of films deposited at substrate temperatures of450° C. and 700° C. were evaluated with the nanoindentation method. Filmthicknesses were approximately 5-6 μm. Indentation tests were carriedout with two different nanoindenters, a Hysitron Triboscope and amicromechanical tester (MMT). The Hysitron Triboscope operating inconjunction with an atomic force microscope combines nanoindentationwith the ability to image the indented area. With the MMT, higher loads(up to 900 mN) can be achieved compared to those available with theHysitron (a few mN). The nanoindentation tests were performed withconical 90-degree indenters having radii of 400 nm and 1 μm for theHysitron and MMT, respectively. Hardness evaluation relied onload-displacement curve analysis using the Oliver and Pharr method.Oliver, W. C., and Pharr, G. M., J. Mater. Res. 7:1564 (1992).

FIG. 9 summarizes the hardness measurements. From this figure, it isevident that hardness values are higher for the high-temperaturedeposit. Even at the relatively high level of 20% porosity, these valuescorrespond to the top of the hardness range reported for fully densestandard SiC. For both films, measurements at shallow penetration depths(Hysitron) indicate that hardness decreases with decreasing depth ofpenetration. This result is consistent with the high surface roughnessand elevated porosity of near-surface layers (observed by SEM).Indentation with the MMT at greater penetration depths yieldsdepth-independent values of hardness. These values are lower than thoseexpected from the low-depth measurement trends.

Qualitative adhesion strength assessment was performed with a 1 kgapplied load using a Vickers Indenter. Higher temperature depositsexhibited better adhesion, and only film cracking was apparent. Incontrast, extensive spalling was observed for a lower temperaturedeposit. Quantitative measurements were not possible in either case, dueto irregular spalling or film cracking. To obtain quantitative adhesionmeasurements, 1 μm tungsten overlayer was sputtered over the SiC films.Tungsten was expected to reduce the effects of indentation-inducedcracking and to increase the force for delamination. Indentation teststo evaluate adhesion were carried out with the MMT. For a lowtemperature film, measurements obtained from delamination induced by a900 mN load indicate an adhesion strength of 2.7-3.9 J/m². Nodelamination was observed for the high temperature film.

A number of embodiments of the invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention.Accordingly, other embodiments are within the scope of the followingclaims.

1. A particle deposition apparatus, comprising: a) a particle sourcethat generates a plurality of particles in a suspended form; b) anexpansion chamber in communication with the particle source and havingan interior pressure that is substantially lower than the interiorpressure of the particle source; c) a deposition chamber; d) a substratesupport located inside the deposition chamber; and e) an aerodynamicfocusing stage connecting the expansion chamber to the depositionchamber, and comprising a plurality of aerodynamic focusing elements. 2.The particle deposition apparatus of claim 1, wherein the particlesource generates a plurality of nanoparticles.
 3. The particledeposition apparatus of claim 1, wherein the particle source includes adc torch.
 4. The particle deposition apparatus of claim 1, wherein theparticle source is a thermal plasma expansion reactor.
 5. The particledeposition apparatus of claim 1, wherein the particle source is a laserpyrolisis reactor.
 6. The particle deposition apparatus of claim 1,wherein the particle source is an evaporation-condensation reactor. 7.The particle deposition apparatus of claim 1, wherein the particlesource is an electrospray particle source.
 8. The particle depositionapparatus of claim 1, wherein the aerodynamic focusing stage comprisesbetween two and five aerodynamic focusing elements.
 9. The particledeposition apparatus of claim 1, further comprising a hollow nozzlebetween the particle source and the expansion chamber.
 10. The particledeposition apparatus of claim 9, wherein the hollow nozzle generates ahypersonic flow of particles, and the aerodynamic focusing stage focusesthe flow of particles into a collimated beam.
 11. The particledeposition apparatus of claim 10, wherein the substrate support ispositioned normal to the collimated beam.
 12. The particle depositionapparatus of claim 1, wherein the substrate support is translatable. 13.A method for depositing particles on a substrate, comprising the stepsof: a) generating an aerosol cloud of particles; b) accelerating theparticles through a nozzle; c) creating a collimated beam of particlesby passing the particle through a plurality of aerodynamic focusinglenses; d) impacting the collimated beam of particles against thesubstrate.
 14. The method of claim 13, wherein the particles arecomprised of nanoparticles.
 15. The method of claim 13, wherein theaerosol cloud is generated in a plasma expansion reactor.
 16. The methodof claim 13, wherein the nanoparticles are accelerated through thenozzle to hypersonic speeds.
 17. The method of claim 13, furtherincluding the step of translating the substrate in a predeterminedmanner to create a pattern of nanoparticles on the substrate.
 18. Themethod of claim 17, wherein the substrate is translated in more than onedirection.